Battery systems have been proposed each of which is provided with a battery panel, a DC/DC conversion device, and an AC/DC conversion device, and charges electric power from an electric power system and discharges power to a DC load and an AC load, and is operated in connection with the electric power system. Each of the battery panels of the battery systems like these is provided with a battery pack composed by combining a plurality of secondary battery cells in accordance with battery capacity.
Since a lithium ion secondary battery cell as the secondary battery cell, for example, is at the risk of heat generation and firing due to overcharge and over discharge, in order to protect the secondary battery cell, it is necessary to operate the secondary battery cell under the supervision of a battery management unit (BMU: Battery Management Unit) to monitor voltage and temperature thereof.
In a battery system having a battery pack composed of lithium ion secondary cells, the battery pack can not be charged and discharged in the state in which the battery management unit is not stated up. For the reason, the battery management unit can not be started up under the circumstance in which electric power is not obtained from outside, and thereby the power charged in the battery system could not be supplied to a load sometimes.